I exchanged several email with RIM tech support and obtained a link to the AT&T website that proved they (AT&T) are responsible for warranty exchanges with the 8820 OEM case. The trick is to find a knowledgeable AT&T rep at a corporate store. Without this combination things seemed dead in the water.

RIM customer service was great!

Thanks to all for their suggestions.

Yeah, if you dig up the warranty information, accessories that originally come with the device are covered under the warranty. So it doesn't matter if they carry it or not for regular retail sales. However, they can substitute.

Tmobile's warranty is extremely poorly written, but I would think AT&T's is a bit clearer.